This invention relates to a scrubbing solution and a process of use thereof for the removal of nitrogen oxides from the effluents of combustion processes. More particularly, this invention relates to a scrubbing solution and process of use thereof for removal of nitrogen oxides from the gaseous effluents of combustion processes utilizing gas-liquid absorption.
As a result of recent concern and regulations regarding air pollution, many attempts have been made to reduce the amount of nitrogen oxides which enter the atmosphere. However, in spite of these attempts, it is estimated that over 20 million tons of nitrogen oxides are emitted to the atmosphere in the United States every year. Of this amount, over half is due to stationary sources such as electric power plants, industrial combustion or gas plants. In these combustion processes, the nitrogen present in the air combines with the oxygen to produce varying amounts of nitrogen oxides, including nitric oxide and nitrogen dioxide.
The continued emission of these nitrogen oxides into the air is of grave national concern, since it has been shown that even low levels of these oxides in the air have detrimental effects on health. Natural background levels of 1 part per billion have been supplemented with an average urban level of 10-50 parts per hundred million. Since stack gas from typical stationary combustion sources, as noted above, contains from 100-1500 parts per million of nitrogen oxides, efforts must be made to reduce the level of nitrogen oxide emissions from these combustion processes, which are necessary to our daily national existence. Furthermore, the nitrogen oxide pollutants react with other pollutants, such as sulphur oxides and hydrocarbons, in the atmosphere to form the now common smog of the larger industrial centers. Although smog was once only a problem of the largest and most densely populated national urban centers, the general background level has been raised sufficiently in past years that it is becoming a problem even in areas of the country which are not industrially developed.
Although there are other sources for the production of energy and power which can eventually replace fossil fuel power stations, it is estimated that it will be a number of decades before these power sources can be economically used and that, until that time, at least 100 and possibly 300 new fossil fuel combustion stations will be built for the generation of electricity alone. Therefore, it is vitally important that a method be developed for controlling nitrogen oxide pollutants from present sources of combustion power and fossil fuel power stations which will be built in the next 20 years. This fact alone has given increased impetus to the research surrounding the elimination of nitrogen oxides from power plants and other combustion process emissions.
There are two general approaches for the elimination of nitrogen oxide emissions from power plant stack gases. One approach is to modify the combustion process so that nitrogen oxides are not produced in sufficient quantities to cause harm. However, this approach is not practical with respect to the numerous power stations which are already in operation, since these modifications would require extensive capital improvements and other changes which would make cost of such procedures prohibitive. The second alternative is, by far, the most practical commercially, since a process for the removal of nitrogen oxides can be adapted to any existing power plant with a minimum of expense and capital improvement. With regard to the second method, namely, the removal of the nitrogen oxide pollutants from the exit streams of these power sources, a number of possible methods for the removal have been proposed: the use of a reducing gas to reduce nitrogen oxides to nitrogen; the use of gas-permeable membranes which are selective for the specific pollutant; the use of an electric arc which, when passed through an atmosphere of nitrogen, produces atomic nitrogen which would react with the nitrogen oxides to produce nitrogen and oxygen; the use of various complexing agents which form complex ions with the nitrogen oxides in the flue gas; the use of solid adsorbants which adsorb the nitrogen oxides from the flue gas; the use of a catalyst with or without a reducing agent; and, finally, the use of a liquid solution to absorb the nitrogen oxides from the flue gas.
The use of solid adsorption to remove nitrogen oxides from flue gases is not a new idea, since well-known processes for the production of nitric acid rely on the adsorption of nitrogen oxides on silica gel. However, although these solids are active and remove the majority of the nitrogen oxides from the flue gases, they are often poisoned by other pollutants and also are easily saturated when the nitrogen oxides are present in excess of 1000 parts per million in the gas stream.